I. Field of the Invention
The present invention relates to a bending machine provided with a blank holder having programmable length.
II. Related Art and Other Considerations
Machines are known for bending metals sheets, called bending machines, that are used for the production of metal sheet panels of a substantially rectangular shape having bent edges. The panels are used, for example, in the manufacture of casings for home appliances, metal furniture, shelves and the like.
Generally, the known bending machines comprise a pair of opposite bending blades, one upper and one lower, a fixed bending counterblade and a blank holder capable of interacting.
The bending blades are supported by arms of a structure, preferably in the shape of a "C", so as to allow movements driven by special mechanisms. The "C"-shaped structure is capable of translating vertically so that one or the other blade is brought to engagement with an edge of the metal sheet to be bent so as to execute a bend upward or downward.
The counterblade is integral with the base of the machine and on it is placed the metal sheet to be bent. It is in a single piece and it has a horizontal edge, against which one of the bending blades urges the metal sheet so as to bend it downward or upward.
The blank holder is used to keep the metal sheet in adherence with the counterblade during the bending operation so that the bend will be straight and with a small radius of curvature. The active edge of the blank holder, above the edge of the counterblade, co-operates with the blade used for bending upward or downward the metal sheet held tight between the blank holder and the counterblade.
In some known bending machines, the blank holder is in a single piece. Such machines can bend a metal sheet only along sides that not intersect one another.
On the other hand, in order to produce panels bent along sides that do intersect one another and have any length, bending machines are used provided with a blank holder having a length that is variable with continuity or with very small steps.
The U.S. Pat. No. 4,532,792 describes a device for adjusting the length of a blank holder comprising a group of first blank holder segments and a plurality of second thin blank holder segments. The first blank holder segments are slidable along a first horizontal axis having a direction transversal with respect to the blank holder, while the second blank holder segments are mounted in an invertible manner in the centre of the group of first blank holder segments. The second blank holder segments have a very small width, substantially less than that of the first blank holder segments. A finger-type driving mechanism causes the first blank holder segments to slide along the horizontal axis and a reverse mechanism drives the reversal of a predetermined number of second blank holder segments making them rotate round a second horizontal axis through an angle of about 180.degree.. In that way, the surplus second blank holder segments are brought to a position at rest and the first and the second blank holder segments that have remained in a work position can be brought closer together so as to restore the continuity of the active edge of the blank holder having the desired length.
This known device has the disadvantage that the second blank holder segments, rotating, occupy part of the space surrounding the blank holder. They therefore move to an area that is generally occupied by a tool-change device or that must remain available for such device. It happens in fact that the sharp edges of the counterblade and of the blank holder do not always meet the requirements of the bending operation. It is therefore sometimes necessary to apply extensions with more suitable profiles. The device that handles such extensions (tool change) and the extensions themselves, especially those destined for the blank holder, given their requirement of rigidity and accuracy and in view of the limited time available for the change, must be quite close to the bending edges and would therefore interfere with any element that were to protrude from the blank holder.
The device of the U.S. Pat. No. 4,532,792 does not allow this constraint to be observed because the second blank holder segments, when they are in an at rest position, occupy precisely a part of the area reserved for the tool-change device.
An object of the present invention is a bending machine provided with a blank holder having programmable length to leave the surrounding space free and not interfere with the tool-change device.